Small island nations, particularly those in the Pacific, are already experiencing "extreme effects" from global warming, and rich nations including Australia have a "moral responsibility" to help them cope with future unavoidable threats, a senior World Bank executive said.

Atoll nations including Kiribati, Tuvalu and the Marshall Islands are seeing shifting rainfall patterns, rising sea-levels and ocean acidification that are forcing islanders to move, said Rachel Kyte, the World Bank's special envoy for climate change.

"For some of the islands, we're really talking about the extreme effects of climate change now, which are going to put their entire cultures in jeopardy within the foreseeable future," Ms Kyte said ahead of the biggest gathering of island nation officials for a decade, starting on Monday in Samoa. Foreign Minister Julie Bishop is due to attend.

For low-lying island states such as Tuvalu, sea-level rise is a serious threat.

"We have an obligation to help build these countries' resilience," Ms Kyte said, not only for economic reasons but also for moral ones. "Some will argue that this is an actual issue of justice and an actual issue of rights" given the role rich nations have played in emitting greenhouse gas emissions to "poison" the atmosphere, she said.

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Climate change will be "a significant focus" of the islands conference, said Pradeep Kurukulasuriya, global head of climate change adaptation for the UN Development Program. "It's at the core of their development issues."

Coastal erosion and inundation are already forcing relocations. For residents in very low-lying states, "there may not be anything else to do but migrate", he said.

Melting ice is making Antarctic waters less salty - and contributing to the unusually rapid sea-level rise. Photo: David Doubilet/ElysiumEpic

A report on the changing Pacific climate released this year by the CSIRO and the Bureau of Meteorology found that since 1951, the frequency of warm days and nights had increased more than threefold across the region.

"Once rare extremes that used to occur approximately 20 days in a year, [they] are now occurring much more frequently, between 45 to 80 days in a year," it said.

Extreme rainfall events that occurred once every 20 years on average during the 1986-2005 period are projected to occur once every seven to 10 years by 2090, even under the best-case low emissions scenario, and every four to six years by 2090 under the worst, the report said.

New Zealand was already discussing taking migrants forced to leave their islands, Ms Kyte said, adding Australia may end up joining in. "We all have an obligation [to help] but good neighbourliness is a good Australian tradition."

While higher sea-levels are an issue in the Pacific, new research suggests Antarctic waters are rising among the fastest in the world because of the increasing rate of melting of the Antarctic ice sheet.

A study published in Nature Geoscienceon Monday has found that from 1992 to 2011, sea-level rise along the Antarctic Coast was at about 4.4 millimetres a year – 1.2 millimetres more than the global average and more than 2 millimetres more than the Southern Ocean overall.

"We know when the water is colder, such as at the poles, that its density is much more influenced by changes in salinity than in temperature, just because it's colder," said Craig Rye from the University of Southampton, lead author of the report.

Salinity levels are falling because of a thinning of Antarctica's grounded and floating ice, with the annual loss in the order of 350 gigatonnes as the planet warms up, according to the report.